1. Field of the Invention
The present invention relates generally to the field of optical fiber multiplexers. More particularly, the invention relates to an optical fiber multiplexer that may be used in spectroscopy applications, such as Raman spectroscopy.
2. Description of Related Art
Raman scattering is a type of spectroscopy which measures very faint light scattering effects. Raman scattering is used to perform qualitative and quantitative chemical analysis of samples. Unlike other types of spectroscopy, the measured Raman light signal is relatively weak. Raman spectroscopy devices typically include a relatively powerful laser light source (200 to 600 mW), means for collecting and detecting the scattered light, and means for measuring and analyzing the scattered light. Optical fibers may be used to transmit an excitation light signal from a laser source to a probe and to transmit a detected scattered light signal from the probe to a Raman spectroscopy device. See, U.S. Pat. No. 5,751,415, U.S. Pat. No. 6,028,667, and U.S. Pat. No. 6,100,975.
Because laser sources and spectroscopy devices are relatively expensive, it would be an advancement in the art to provide a multiplexer device so that one laser source and spectrometer device can be used to analyze multiple chemical samples which are connected to the multiplexer with optical fibers. There is a need for such a multiplexer in industrial and research laboratory settings.
Fiber optic multiplexers used in spectroscopy are known and commercially available from Equitech International Corporation (www.equitechintl.com) and Piezosystem Jena, Inc. (www.piezojena.com). Both of these manufacturers use direct fiber-to-fiber coupling in their multiplexers. Butting one fiber against another is effective only for relatively low power applications. The high power required for Raman spectroscopy would create excessive power density at the end of the fiber. Under such circumstances, dust or other contamination at the end of the fiber can be vaporized, leaving an attenuating film to block and diminish the light signal transmission. Another disadvantage of fiber-to-fiber coupling is that it requires very accurate alignment of the fibers, something which may not be possible with repeatedly moving fibers. Thus, there is a need in the art for an effective optical fiber multiplexer that does not require fiber-to-fiber coupling.
Piezosystem Jena utilizes piezo motor control to move its optical fibers very small distances. One fiber can be switched between at most two or three fibers. Piezo motor systems cannot be used with large diameter fibers, such as, 400-1000 μm fibers. In order to have an 1.8 channel device it would be necessary to have three or more successive couplings. Each time a fiber is coupled to another fiber, transmission losses of 15% to 20% occur. This means that from 45% to 60% or more of the original Raman signal would be lost using a piezo motor system. The number of times a signal is coupled must be minimized with Raman spectroscopy since the original source signal (scattered light) is so weak, compared to other spectroscopy techniques. Thus, the Piezosystem Jena approach cannot be used effectively with a multi-channel device for Raman spectroscopy which requires large diameter fibers or requires more than about three channels.
The Equitech International system uses a worm gear to move a rotating plate. Worm gears are not desirable because they have a certain amount of slack (backlash) in the gear connection which results in accuracy problems. Moreover, over time, the gears wear and introduce additional alignment error. Backlash may be eliminated by pre-loading gears, but this would introduce severe gear wear and other problems. Similar errors arise due to the need for periodic lubrication of the gears. Thus, it would be an advancement in the art to provide an optical fiber multiplexer device that does not use a worm gear or similar gear-driven mechanism.